This invention relates to a method of manufacturing an X-ray mask used in X-ray lithography.
Recently, remarkable integration has been accomplished in a very large scale integration circuit or the like, with development of a semiconductor technique. Under the circumstances, it is a recent trend that X-ray lithography is used instead of conventional photolithography so as to transfer a fine pattern with a high precision onto a semiconductor wafer. With this X-ray lithography, exposure is made in the X-ray lithography by the use of an X-ray which has a wavelength shorter than light, such as ultraviolet rays or the like. Therefore, the X-ray lithography makes it possible to delineate a fine pattern, as compared with the photolithography.
In such X-ray lithography, an X-ray mask is inevitably used to delineate such a fine pattern. The X-ray mask usually comprises a substrate, an X-ray transmission layer on the substrate, and an X-ray absorption layer on the X-ray transmission layer.
The X-ray transmission layer may be formed either by a single film or by a plurality of films and should have the following properties in addition to high transmittivity against the X-ray. Namely, the X-ray transmission layer must have:
(1) a weak tensile stress as an internal stress; PA0 (2) a characteristic strongly withstanding X-ray radiation carried out on exposure and consequently an invariable characteristic against the X-ray radiation; PA0 (3) a desired flatness or roughness; PA0 (4) excellent chemical stability; PA0 (5) high transmittivity against visible light; and PA0 (6) excellent mechanical strength.
In addition, the X-ray absorption layer must also satisfy various kinds of requirements which are similar to those exemplified above in connection with the X-ray transmission layer. Moreover, the X-ray absorption layer must have a layer structure which is suitable for delineating a fine pattern and which can therefore be readily etched. Therefore, it is preferable that the X-ray absorption layer has an amorphous or a fine crystal structure.
A wide variety of attempts have been made in order to obtain the above-mentioned properties. For example, various kinds of materials, such as silicon carbide (SiCx), silicon nitride (SiNx), boron nitride (BN), or silicon, have been used as materials of the X-ray transmission layer. In addition, deposition methods have been proposed to deposit the X-ray transmission layer and may be, for instance, decompressed chemical vapor deposition, plasma chemical vapor deposition, or sputtering. Such deposition methods may be carried out within various atmospheres.
However, it has been very difficult to deposit a desired X-ray transmission layer which satisfies all of the above-mentioned properties. This is because improvement of either one of the properties often brings about a difficulty of improving the other properties. For example, the internal stress preferably falls within a very narrow preferable range between 2.times.10.sup.8 dyn/cm.sup.2 and 8.times.10.sup.8 dyn/cm.sup.2. In order to adjust the internal stress to the above range, restriction should be strictly imposed on deposition of the X-ray transmission layer. However, such restriction makes it difficult to select conditions for the other properties.
Herein, a conventional thought is rarely directed to the flatness or roughness of the X-ray transmission layer. According to the conventional thought, it has been imagined that the flatness of the X-ray transmission layer has not been significant so much to determine a whole characteristic of the X-ray mask. In this situation, it has been considered that the X-ray transmission layer may have the flatness of 500-1000 angstroms or so.
In other words, consideration has not been made at all about improvement of the flatness in the X-ray transmission layer.
According to the inventor's experimental studies, it has been found out that an excellent X-ray absorption layer can not be obtained when the X-ray transmission layer has the flatness between 500-1000 angstroms or so. Especially, the flatness and the internal stress of the X-ray absorption layer are deteriorated in dependency upon the flatness of the X-ray transmission layer.
In addition, the internal stress of the X-ray transmission layer can not be controlled within the preferable range even when the flatness of the X-ray transmission layer is adjusted to the extent of 500-1000 angstroms or so. As a result, degradation of the flatness of the X-ray transmission layer brings about deterioration of the flatness and the internal stress of the X-ray absorption layer.